Salicylic acid (SA) serves as an endogenous phytohormone in the deployment of Systemic Acquired Resistance (SAR), a broad-spectrum and long-lasting immune response activated by avirulent pathogens in plants. Its deployment is monitored through the marker gene PR-1, whose activation requires the recruitment of an SA-dependent transcriptional enhanceosome to its promoter. The enhanceosome contains members of the TGA2 clade of bZIP transcription factors and the transcriptional coactivator NPR1, which is the central regulator of SAR and SA-dependent gene activation. TGA2 is a transcriptional repressor and thus requires a coactivator to effect gene activation. NPR1 provides a dual function in the enhanceosome. First, its N-terminal region contains a BTB/POZ domain which interacts with and negates the function of the TGA2 repression domain, and secondly, NPR1 harbors in its C-terminal region a transactivation domain, which contains two cysteines (Cys521 and Cys529) required for the activating function of the enhanceosome.
In non-SA-stimulated cells, NPR1-GFP fusion proteins behave as oligomers on sodium dodecyl-sulfate-polyacrylamide gel (SDS-PAGE) electrophoresis. Endogenous NPR1 localizes to both the nucleus and the cytosol and nuclear localization is critical to PR-1 activation. A fraction of the nuclear NPR1 population acts as a latent coactivator which is recruited under non-inducing conditions to the PR-1 promoter. There thus exists an uncharacterized mechanism by which the NPR1 transactivating domain remains occluded under non-inducing conditions and gets unveiled during SA-dependent gene activation. Furthermore, although genetic analyses have revealed many genes involved in SA-signaling, the receptor responsible for sensing SA and leading to direct or indirect NPR1 activation remains elusive.
While enzymes, such as catalase, peroxidase, and methyl-salicylate esterase, have been shown to directly interact with SA, their proposed role in SAR has not been unequivocal. SA was originally portrayed as a catalase and peroxidase inhibitor, leading to the generation of H2O2, and the production of PR protein. However, H2O2 was later shown not to be a second messenger acting downstream of SA, invalidating the role of catalase and peroxidase as SA-receptors for PR gene activation. Whereas methyl-salicylate esterase has been shown to play a role in tobacco, it clearly has no role in SAR in Arabidopsis. Most importantly, these enzymes do not figure as classical transcription regulators and therefore, they are unlikely to regulate gene expression directly.
It would be desirable to develop novel methods of activating the immune response in plants in order to enhance immunity in plants.